The invention relates to a transmitter, notably a mobile radio transmitter, for generating a transmission signal, which transmitter includes a first modulation device for generating an uncompensated transmission signal by modulation of at least one baseband signal with a respective oscillation signal.
The invention also relates to a method of operating the transmitter.
A transmitter of this kind is known, for example as the integrated circuit PMB 2200 manufactured by Siemens AG. The transmitter includes a modulation device; more exactly speaking, it includes a quadrature modulator as shown in simplified form in FIG. 5.
The quadrature modulator shown in FIG. 5 includes two modulators 52a, 52b, each of which multiplies a respective baseband signal I(t), Q(t) by an associated oscillation signal XLOQ, XLOI.
The baseband signals I, Q concern Cartesian components of a rotating pointer A as is illustrated in FIG. 6. This Figure shows, by way of example, that the baseband signal I is a cosine oscillation and that the baseband signal Q is a sine oscillation, both oscillations being of equal amplitude A. An amplitude of equal absolute value is specified, for example, in the mobile radio standard GMSK.
The oscillation signals XLOI and XLOQ shown in FIG. 7 are binary switching signals that have been phase shifted 90xc2x0 relative to one another.
The first modulator 52a in FIG. 5 modulates the baseband signal Q(t) with the first oscillation signal XLOQ and the second modulator 52b modulates the baseband signal I(t) with the oscillation signal XLOI. The outputs of the two modulators are additively combined in the addition device 54 so as to form a transmission signal Y(t).
Because the oscillation signals shown in FIG. 7 are binary switching signals, they can be decomposed into a fundamental wave and a plurality of harmonics by way of Fourier transformation. The following expression is then obtained for the oscillation signal XLOI:                               x          L0I                =                              cos            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    -                                    1              3                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            3            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    +                                    1              5                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            5            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    -                                    1              7                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            7            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                                              (        1        )            
and for the oscillation signal XLOQ there is obtained                               x          L0Q                =                              sin            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    +                                    1              3                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            3            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    +                                    1              5                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            5            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                    +                                    1              7                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            7            ⁢                          xe2x80x83                        ⁢                          ω              c                        ⁢            t                                              (        2        )            
Thus, the transmission signal Y(t) at the output of the quadrature modulator shown in FIG. 5 is:                     "AutoLeftMatch"                                                                              y                  ⁡                                      (                    t                    )                                                  =                                                                                                                              X                          LOI                                                ⁡                                                  (                          t                          )                                                                    ·                      cos                                        ⁢                                          xe2x80x83                                        ⁢                                          ω                      m                                        ⁢                    t                                    ⁢                                      +                                          (                      -                      )                                                        ⁢                                                                                    X                        LOQ                                            ⁡                                              (                        t                        )                                                              ⁢                    sin                    ⁢                                          xe2x80x83                                        ⁢                                          ω                      m                                        ⁢                    t                                                                                                                          =                                                                            cos                      ⁡                                              (                                                                              ω                            c                                                    ⁢                                                      -                                                          (                              +                              )                                                                                ⁢                                                      ω                            m                                                                          )                                                              ⁢                    t                                    -                                                            1                      3                                        ⁢                                          cos                      ⁡                                              (                                                  3                          ⁢                                                                                    ω                              c                                                        ⁡                                                          (                                                              -                                +                                                            )                                                                                ⁢                                                      ω                            m                                                                          )                                                              ⁢                    t                                    +                                                            1                      5                                        ⁢                                          cos                      ⁡                                              (                                                                              5                            ⁢                                                          ω                              c                                                                                ⁢                                                      -                                                          (                              +                              )                                                                                ⁢                                                      ω                            m                                                                          )                                                              ⁢                                          t                      (                      4                      )                                                                                                                              (        3        )            
This expression for the transmission signal Y(t) is obtained while taking into account the following equations and addition theorems:
Y(t)=Acosxcfx86(t)xc2x7cosxcfx89ct(xc2x1)Asinxcfx86(t)xc2x7sinxcfx89ctxe2x80x83xe2x80x83(5)
                              cos          ⁢                      xe2x80x83                    ⁢          α          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          β                =                              1            2                    ⁡                      [                                          cos                ⁡                                  (                                      α                    -                    β                                    )                                            +                              cos                ⁡                                  (                                      α                    +                    β                                    )                                                      ]                                              (        6        )                                          sin          ⁢                      xe2x80x83                    ⁢          α          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢          β                ⁢                  xe2x80x83                =                              1            2                    ⁡                      [                                          cos                ⁢                                  xe2x80x83                                ⁢                                  (                                      α                    -                    β                                    )                                            -                              cos                ⁡                                  (                                      α                    +                    β                                    )                                                      ]                                              (        7        )            xe2x80x83Y(t)=Acos (xcfx89ct(∓) xcfx86(t))xe2x80x83xe2x80x83(8)
Equation 4 shows that the transmission signal Y(t) includes a plurality of harmonics, notably a third harmonic cos(3xcfx89c (∓)xcfx89m)t, in addition to the fundamental wave cos(xcfx89cxc2x1xcfx89m)xc2x7t. As appears from the equations 1 and 2, the harmonics in the transmission signal Y(t) are caused by the harmonics in the oscillation signals XLOI and XLOQ. FIG. 8 shows a diagram illustrating the amplitudes of the individual harmonics in relation to an amplitude of the fundamental wave that has been normalized to 1. It appears that the third harmonic has the highest amplitude of all harmonics, the absolute value of its amplitude amounting to one third of the amplitude of the fundamental wave; this means that it has been attenuated by only 9.54 dB with respect thereto.
Because the transmission signal Y(t) includes not only the fundamental wave but also the harmonics, it is uncompensated to this extent. The harmonics in the transmission signal are undesirable and were eliminated thus far by low-pass filtering at a later stage. For specific applications in the field of mobile radio, however, attenuation of the third harmonic at the output of the addition device 54 by 40 dB is necessary; this can be achieved only by means of a substantial amount of filter means. A filter that is suitable for this purpose can thus far be realized only as an external circuit for an integrated transmitter. Therefore, integration of the modulation device and the filter on one chip was not possible thus far.
On the basis of this state of the art it is an object of the present invention to improve a transmitter and a method of the kind set forth in such a manner that the expenditure for filtering of the transmission signal at a later stage is reduced.
This object is achieved as disclosed in claim 1 as well as by the method disclosed in claim 9.
For the transmitter of the kind set forth this object is achieved in conformity with claim 1 in that there are also provided a second modulation device for generating a compensation signal, representing essentially the components to be compensated in the uncompensated transmission signal, by modulation of the baseband signal with a suitable auxiliary oscillation signal, and a subtraction device for subtracting the compensation signal from the uncompensated transmission signal and for delivering an at least partly compensated transmission signal.
A transmitter thus constructed offers the advantage that undesirable harmonic components in the generated transmission signal are at least partly compensated. The compensation can become manifest, for example, as a significant reduction of the amplitude of individual harmonic components. The expenditure required for a filter that succeeds the transmitter and serves to attenuate further harmonic components still present in the at least partly compensated transmission signal is then substantially less than for the uncompensated transmission signal according to the state of the art.
When the compensation according to the invention involves notably the compensation of the dominant harmonics within the transmitter, a downstream filter can even be integrated on one chip together with the transmitter.
According to a first embodiment of the invention it is advantageous to choose the amplitude of the auxiliary oscillation signal to be such that the compensation signal generated on the output of the second modulation device has a desired amplitude.
As an alternative, it is advantageous when the amplitude of the compensation signal can be adapted to a desired level by way of an attenuation device.
The oscillation signal and the auxiliary oscillation signal can be particularly simply generated when use is made of circulating shift registers.
It is particularly advantageous when the compensation signal represents essentially the third harmonic of the uncompensated transmission signal, because this harmonic is dominant over other harmonics when binary oscillation switching signals are used and is most difficult to eliminate in a downstream filter because of the fact that its frequency is so close to the frequency of the fundamental wave.
A low-pass filter succeeding the transmitter can be advantageously used for the attenuation of harmonic components still present in the transmission signal after the compensation.
When n modulators and nxe2x88x921 addition devices are used each time in the first and the second modulation device, a plurality of n baseband signals can be modulated and used to generate an at least partly compensated transmission signal.
The object is also achieved by means of a method of generating a transmission signal. The advantages of this method correspond to the advantages mentioned thus far for the above transmitter.